This invention relates to fine crystalline powder of a piezoelectric ceramic composition of the lead zirconate-titanate system, a process for producing the powder, high-density piezoelectric ceramics made of the powder, and a method of manufacturing the piezoelectric ceramics. The piezoelectric ceramics according to this invention are suitable particularly to ultrasonic transducers, base plates for elastic surface waves, and elements for electrooptical applications. The piezoelectric ceramic composition may include, in place of parts of zirconium and titanium, manganese and antimony, niobium, and/or tantalum to remarkably improve the piezoelectric activities as described in U.S. Pat. No. 3,699,045.
Conventional lead-zirconate-titanate-system piezoelectric ceramics are generally manufactured by sintering. The sintered density of the conventional piezoelectric ceramics is about from 95 to 97% of the theoretical value. It has been very difficult, if not impossible, to provide piezoelectric ceramics whose sintered density is 98% or more of the theoretical value although piezoelectric ceramics of this high density are desirable in view of the reduced number of voids within the sintered bodies and in view of the resulting higher mechanical strength and higher smoothness of the mirrored surfaces.
Hot pressing known in the art of powder metallurgy may be resorted to on providing the piezoelectric ceramics of higher densities. The hot pressing, however, is not suitable to large-scale production at present. Alternatively, use may be made as the starting material of active powder of very fine particle sizes as reported by L. M. Brawn and K. S. Mazdiyasmi in "Journal of the American Ceramic Society," Volume 55, No. 11 (November, 1972) , pp. 541-544. The fine powder is produced in general by preparing an aqueous solution of inorganic or organic compounds of the desired metallic or positive elements, subjecting the compounds to hydrolysis to make colloidal hydroxides of the metals deposit as precipitate, separating the precipitate from the mother liquor, and heating the separated precipitate to 500.degree.C or higher to subject the same to decomposition and/or crystallization. It has, however, been inevitable that crystals of the pyrochlor type, harmful to manufacture of piezoelectric ceramics of the high sintered density, are produced besides the desired crystals of the perovskite structure. It has also been very difficult to carry out the separation because of the extreme fineness of the precipitate. In addition, it has been impossible to prevent the precipitate from missing from the resultant, which fact has often resulted in fluctuation of the compositional ratios.